The present invention relates to a process for preparing very viscous, heat-stable poly(butylene terephthalate) by condensation of terephthalic acid, or a lower alkyl ester thereof, with 1,4-butanediol in two stages at an elevated temperature and in the presence of a conventional catalyst.
Poly(butylene terephthalate) (PBTP) is an excellent material for the production of films, sheets, moldings and the like. For producing moldings by injection-molding and extrusion, in particular, PBTP should have a reduced specific viscosity of &gt;1.0 dl/g (measured at 25.degree. C. in a solution of 0.23 g of PBTP in 100 ml of a solvent mixture of phenol and tetrachloroethane in the weight ratio of 60:40).
Usually, PBTP is prepared by first trans-esterifying dimethyl terephthalate with an excess of 1,4-butanediol, in the presence of a conventional catalyst, under normal pressure, to produce an oligo-ester; and then removing the excess 1,4-butanediol under reduced pressure and at an elevated temperature, in the course of which a polycondensation of the oligo-ester formed during the trans-esterification takes place (J. G. Smith et al., J. Polymer Sci. (A) 4, 1,851 (1966); Kunststoff-Handbuch (Plastics Handbook), volume VIII, C. Hanser Verlag Munich, edited by L. Goerden).
Using these conventional processes, the high viscosities that are required, for example, for extrusion processes cannot be achieved. The viscosity is limited since degradation reactions already start due to the relatively long heat exposure during the polycondensation. Thus, a certain viscosity cannot be exceeded. Thus, in order to achieve the high viscosities needed, for example, for extrusion applications, a solid phase post-condensation of the PBTP is therefore usually additionally carried out (British Pat. No. 1,066,162; U.S. Pat. No. 3,405,098).
However, the solid phase post-condensation is a technically demanding process, frequently exhibiting undesirable concomitant phenomena, such as, for example, a deterioration of the intrinsic color of the polyester.
Another possible way of increasing the viscosity of these polyesters is by co-condensing trifunctional or polyfunctional alcohols, carboxylic acids or their esters (German Auslegeschrift No. 1,614,980). However, it is difficult to control this reaction so as to avoid crosslinking. Furthermore, this process easily leads to products containing specks.